Charge Amplifier Bias Decoupling for MEMS Sensor Dynamic Range
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Solution Overview
Problem
MEMS microphone readout circuits suffer from high distortion due to voltage clipping as the amplitude of acoustic signals increases, limiting the dynamic range and acoustic overload point (AOP) of the output signal.
Innovation Solution
A charge amplifier circuit design that decouples the biasing voltages of the input and output nodes using a control circuit and level shifter, allowing the output biasing voltage to be directly proportional to the supply voltage, thereby preventing clipping and maintaining a high dynamic range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stress or pressure
If the output biasing voltage is set close to the supply voltage to maintain high input impedance, then the input impedance is improved, but the output voltage clips at high signal amplitudes reducing dynamic range
Solution Approach 1:
The patent segments the biasing voltage function by introducing a separate output biasing voltage (VCMO) that is decoupled from the supply voltage VDD through a level shifter circuit. This allows the input node to be biased at a voltage close to VDD for high input impedance while the output node is biased at a lower voltage that prevents clipping, thus resolving the contradiction between input impedance and dynamic range.
Solution Approach 2:
The patent introduces a level shifter circuit as an intermediary component between the supply voltage source and the output biasing voltage. This level shifter decouples the output biasing voltage from direct dependence on the supply voltage, enabling independent optimization of input impedance (through VDD-coupled input biasing) and dynamic range (through level-shifted output biasing that prevents clipping).
2Measurement precision
If the feedback resistor value is increased to improve noise performance, then the noise performance is improved, but the output voltage clips at lower signal amplitudes reducing the acoustic overload point
Solution Approach 1:
The patent changes the output biasing voltage parameter (VCMO) to be directly proportional to the supply voltage through a controllable level shifter, rather than being fixed or indirectly coupled. This parameter change allows the system to maintain high feedback resistor values for low noise while preventing output clipping, thereby improving both noise performance and acoustic overload point simultaneously.
3Reliability
If the supply voltage is increased to expand the output dynamic range, then the output dynamic range is improved, but the output voltage clips at higher amplitudes increasing distortion
Solution Approach 1:
The patent segments the voltage scaling function by introducing a controllable level shifter that decouples the output biasing voltage from direct proportionality to the supply voltage. This allows the supply voltage to be increased to expand the output dynamic range while the level shifter adjusts the output biasing voltage to prevent clipping, thereby eliminating distortion even at higher supply voltages.
Solution Approach 2:
The patent employs a control circuit that provides feedback control of the level shifter to maintain the output biasing voltage at an optimal level. This feedback mechanism ensures that as the supply voltage changes, the output biasing voltage is adjusted accordingly to prevent clipping and distortion, allowing the system to utilize the full dynamic range without generating harmful distortion products.
Data Source
AI summary
A charge amplifier circuit is provided. The charge amplifier circuit is couplable to a transducer that generates an electrical charge that varies with an external stimulus. The charge amplifier circuit includes an amplification stage having an input node, couplable to the transducer, and an output node. The amplification stage biases the input node at a first direct current (DC) voltage. The charge amplifier circuit includes a feedback circuit, which includes a feedback capacitor, electrically coupled between the input and output nodes of the amplification stage. The feedback circuit includes a resistor electrically coupled to the input node, and a level-shifter circuit, electrically coupled between the resistor and the output node. The level-shifter circuit biases the output node at a second DC voltage and as a function of a difference between the second DC voltage and a reference voltage.